This invention relates to providing an independent memory for microprocessor controlled recloser controls. An example of a microprocessor controlled recloser control is provided by U.S. Pat. No. 4,535,409 filed Sept. 18, 1981 of which I am an inventor. That application is in its entirety hereby incorporated by this reference into this application.
Generally, in an electric power distribution system, there are a great number of protective devices for interrupting excessive current flows which tend to damage the current conductors or other distribution equipment. An examination of a system would generally disclose an upstream protective device close to the power source capable of interrupting the entire load served by the power source. The upstream device is generally suceeded by a number of downstream devices of sucessively lesser interrupting capability, each being capable of interrupting the load they protect. It is most desirable to interrupt a fault, excessive current, with the protective device which is closest to the malfunction causing the fault in order to minimize the area of power outage. To this end, a number of protective devices may be placed in series, to conduct current to a load. Then when a fault occurs, only the one closest to the fault should operate to interrupt the fault.
It is also desirable that a protective device respond quicker to faults of greater magnitude than to faults of lesser magnitude. However, it is not desirable that a protective device respond quickly to every current in excess of design limits, since such currents may often be caused by a malfunction of such short duration that no harm to the system will occur. A branch striking a power line due to a gust of wind can cause such a malfunction. Therefore, most protective devices have time current characteristics which delay their response to less excessive current and yet accelerate their response, so that they respond more quickly, to faults of greater magnitude.
A graph of the time current characteristic, showing the time it takes for the device to respond to a fault of a specific magnitude, often results in a straight line when plotted on a log-log scale. Some care should be taken to coordinate among protective devices in a system so that the time current characteristic of an upstream protective device is always greater, i.e. responds more slowly, than a downstream device. Unless such care is taken, a larger area than necessary will suffer power outages needlessly.
Unfortunately at high magnitudes of fault current, the time current characteristics of many protective devices tend to merge. At these levels of fault, coordination among protective devices can be lost. When coordination is lost, several protective devices may respond, or an upstream protective device may respond before a downstream device. To ensure that coordination is not lost under these circumstances, a minimum time delay may be introduced for each of the devices, so that despite merger of their time current characteristics, the device furthest upstream will be the last to open because it has the longest minimum time delay. Due to the complexity of modern power distribution systems and despite precautions, it is not always possible or desirable that coordinations be preserved.
Reclosers are inserted into power lines to protect a power distribution system. Most faults on power distribution lines are of momentary nature and of sufficient magnitude to blow fuses if allowed to be conducted by them for a sufficient period. When a fuse does blow in a power distribution system, it is necessary to send somebody to change it which is a somewhat expensive proposition. A primary function of a recloser is to save fuses. In general, this is done by sensing the peak value of the current conducted and interrupting its flow by opening or tripping a recloser before the fuses can blow. After an interval, the recloser closes and restores power to the system where it remains closed until the next fault is sensed. The rate at which a fuse will blow and interrupt current is a function of the thermal heating of the fusible element. The rate of thermal heating is proportional to the power generated by the fault and each fuse has a time current characteristic which describes the time interval required to interrupt the fault current. The time interval is generally approximately inversely proportional to the value of the root mean square, squared of the fault current. It is desirable to coordinate the recloser with the fuses to be saved to insure that the recloser in fact interrupts temporary fault currents before the fuses to be protected are blown. This is generally done by approximating the root mean square value of the fault current by sensing its peak value.
It must also be recognized that all faults which occur on a power distribution line are not temporary, such as those caused by a branch momentarily falling against the line. Some faults are of a more permanent nature such as those caused by a line falling to the ground. As a consequence, reclosers are built so that they will only trip a limited number of times within a short duration before locking open. Were this not done a recloser would cycle until failure and many of the fuses to be protected would blow anyway. At some magnitude of fault current it is desirable to have the recloser open immediately to protect the line rather than following a time current characteristic. At intermediate levels, it may be desirable from the power distribution standpoint to allow the fault current to flow for a limited period to allow the fault to burn itself open or blow the fuse. Many reclosers have alternate time current characteristics which achieve this goal. Typically a recloser will allow two shots or trip operations to follow a fast time current characteristic and two additional shots along a somewhat slower time current characteristic before locking open or out.
Microprocessor based recloser controls possess a number of advantages which were lacking in the electronic recloser controls which were their forerunners. Most advantages relate to their flexibility and self-diagnostic abilities which largely result from their ability to store and use comparatively great amounts of information in their memory. However since microprocessor based recloser memories are controlled by processing means, the memories cannot be altered unless sufficient power is being supplied to the microprocessor to allow it and related devices to function reliably. Power supplies for recloser controls typically include batteries as energy storage means. However the ability to supply power to a microprocessor in a recloser control for extended periods of time when an upstream device has interrupted power to its power supply is limited. As a result, events can occur when the microprocessor is disabled which are not remembered by the microprocessor associated memory.
A particular sequence of events illustrates one drawback to the inability of microprocessor memory to function during periods of extended power outage. A given recloser trips due to a temporary fault condition. As the recloser control is attempting to reclose its battery lacks sufficient reserve to allow the microprocessor to reliably function and the microprocessor is disabled, aborting the reclosing attempt. Power is restored to the system but, the recloser remains open because the control has initialized its state and conformed to the state of the recloser. It is necessary that the control conform to the recloser to avoid inconsistent operation of the recloser. The result is a power outage downstream of the recloser when no fault condition exists.
An alternative to the outage would be to close the recloser on every occasion that the control is resupplied power. In the past a similar approach was taken but the prior control as a whole was either functional or non functional and it remembered intervening events. With a microprocessor control it is desirable to only disable the processor means, while retaining basic control functions independent of the microprocessor. If during the duration of power outage a secondary overcurrent trip means trips the recloser or the utility desires to change the distribution path by activating a lockout signal, it would not be desirable to close the recloser when power is resupplied to the recloser control.